Separation of recovery of xylene isomers



Jam 15, 1957 M. c. HOFF ETAL SEPARATION OF RECOVERY OF' XYLENE ISOMERSFiled May l. 1953 INVENTORS! Me/vem 0. Hoff By Kennet/r 6. PetersonATTORNEY 2,777,8818 A,SEPARA`1`IO1`I `.0 F RECOVERY 0F AXYLENE 'ISOME'RSMelyerniC. Holi, Highland, and Kenneth C. Peterson, Hammond, Ind.,assignors to Standard il.:;C,ompany, Chicago, lll, za corporation ofIndiana Application May 1, 195s, sei-iai NQ. s525434 s claims..(01.260474) This invention .relates to a process .of lrecovering andseparating xylene isomers especially from mixtures of thefsame withethylbenzenefand .parains boiling insubstantialily the same range. Ilheinvention ihas particular .reference to aprocess for separating.individual stylene isomers from eaoh other and .from such-mixtures 'byAfraotior-ia'l .crystallization Xylenes arefound insubstantial.quantitiesin cokeoven ylight `oil and .certain -vi-rginlandreformed'petroleu-m naphthas. 7It h-as been the practice .in separatingXylene isomers either to use chemical methods lor to -distill'lparaandmeta-xylene, ethylbenzene, and some .parali-nsl from a 'fractioncontaining the isomers, ethylbenzene and -parai-ns, thus separatingortho-Xy'lene as a distillation `bottomsproduc-tin admixture with someof the para-illus, and-to separate para-xylene from the vdistillate-byfractional crystallization. Methods have been devised which do notrequire a'distillation -stepfbutthese' provide yfor separation of lonlythe single isomer, parasry-l'ene, andin a yield limi-tedio only thatamount -theoreticallyobtainable in a single step from a mixture ofxylenes, ethylbenzene and parall-ns, as governed 'by the phaseequilibria as diagrammed, AJfor example, by Kravehenko in- ActaPhysicoc-lrim'ica (U. R. 'SL 8.), vol. 20, 567477( 1%945) for a mixtureof the xy-lene isomers and ethylbenzene. It is a primary object of thepresent invention to provide a method -for the recovery and separationof eaohof A,the vseparate -xylene isomers solely by a system offractional crystallization of a mixture of rthe isomers, ethylbenzeneand p arains. It is a further object of the invention to recoverindividual Xylene isomers, such as para-xy-lene and ortho-xylene, inyields substantially inexcess .oft-hat `theoretically obtainable by asingle fractional crystallization according to -the phase relationshipofthey sys-tem. The invention has 4,for other objects such other advan'tages orresults as will -be'ffou'nd -in the speciicationand claimshereinafter made.

The present invention comprises a crystallization system which consistsof cooling a mixture of-the xyiene isomers and removing the separateisomers, in substantially pure form, in several crystallization stages,usually induced by seeding., while retaining other isomersor eutecticmixtures in supercooled solution, in every case shift-ing to thecrystallization of another isomer before coolingvdown as low as thespontaneous crystallization temperamre of any supercooled component.After crystallization and separation from the mot-her liquor ofpara-Xylene, which substantially always isr the first isomercrystallized from a crude mix-ture of -X-ylene isomers, the secondcomponent is removed; lThe mother liquor is cooled to Va `ternlgreraturebelow the equilibrium crystallization temperature of theA secondcomponent to lue-removed but above the spontaneous .crystallizationtemperature of any .other .component .of the rnixtureand sseeded with acrystal of the-.Second component; At this -point the mother l-iquormaybe Sllpfsatlllatedwih badi .other isomers in whit-:h .case theSeedsdcomnoneut is the one which crystallizes. After crystallization ofprovides secondary.

2,777,888 Patentedl Jan.. 15'. 1957 .aided-bvseediug thissecond',motherAliquor (from @the paraand :nexf-removedfmmponerrt crystallizaticms')v.with al crystal of the third .isomen usually meta-Jn/leue.A

The lmixture .of :xgylene isomers, ethylbenzene and paraftins .boilingnfthegsame boiling-range is ;cooled to a ytem- Parature .between :about-7:5 and 90 C., and isA maintained at this temperature-for a Asuleieu-ttimeto effect crystallization of. the :parafxylene in an amount largerthan the excessof paraaylene overzitsteutectic :proportion inthemixture., thusitakingfadvantage of the-supercooling properties of'onthof .andimeta-xylene. Thev nsualiequilibrium4 aducen-'tration.ofrxylenemixtures obtained from ,the reforming -tof'oetroleum is ,suchthat'para-.xylene and usually orthomylene s-,fin excess of ,the ternaryVeutectic minute, i. e.,unonzcoolingpara-Xylenewill tirst .crystallizeout, andupon further cooling, Aczrtho-xylene and .later metarxylene willbecome saturated or'supersaturatediand ilsocrystalilizo All offthefxylene isomers will supercool .andiofxthefisomera met'aexyleneshows the strongest super- .coolingfcharacteristics Thus, veven ywhenlrneta-xylene is prieseiltin"afproprtion greater than its ternary.eutectic proportion i-t willi stay in isolutionneyen after`ortho-xylene, when needed, isfcrystallized .ou-t.

in 'operation .of .our invention, therefore, when the spontaneouscrystallization vtemperature of ortho-xylene ls approached' .(:in theyequilibrium compositions, about .-\-9G"'.C.):, in which caseusuallyVabout two-th`irds .ofxthe paraxylene has beennr'ecovered, y.theparasyllene crystals .arexseparatedi from she mother'liquor andthemother ,liquor Pfrom 4para-.nylane .crystalsfhave been substantiallycompletely separated is 7then. .cooled and seeded lor' :inoculated:with. onthoosylene` to el'ecty Aa ,crystallization of :this isomer.

Ortho-xylene is crystallized` :by cooling.' .the mother liquorito yatemperature between aboutl e8=5v9 andre-95@ C. and preferably betweenaboutgvzSSthand-Qi C; and seeding ithe .cooled mother liquor with:anortho-xylene crystal. `fllisseooling:iofnthe mother lliquor willefect-a supercooling with respect to ,para-Xylone--and lmetaxyilenebutwthe minimum temperature reached-by=th`s coolingYofvthemother-iliquor-'is floept. abone the spontaneous crystallization.temperature of; paravormetaorylene, usually abouty .1994?" Iowa-98a C.'After sepa-.rationfof ortho-Xylene crystalsy otromlthe mother liquor,theeyole -can be frcpeatedfand this secondary: mother liquor from theortho-xylene .crystallization'fisf'cooled to 'a temperature hetweenabout.-71985 andw'l? TC; and-lis seeded withparafxylene. "'l`his-coolingSandinoculation step'fwi-ll effect the l crystallizatiomofsa .substantialladditional quan- Iity .ofrpure para-xylenehecause theintermediateremoval of .the `next saturated xylenefisomenfusuafllyr orthoexylene,mother liquor froml which-further vquantities 4ofpara- Xyl'ene cam-be:crystallized beforethe mother liquor becomes so saturated with Irespect-to another. isomer thatfits .spontaneous crystallization vter-nperature'fwill haveY 'been reached. `They additional .-para-xy'lene recovered'in thislmannerl isJsepara-tedfrom :the mother liquor ofV.th-isfcrystallization; this second crystallization ofiparaaxylene vlowers Fits concentration in ,the mother liquor so thatzorthoxylenefeanagain be separatedby a second crystallization `.oft-hisrisomer. The-rnotherliquor from :the second `crystallization .of :para-xyleneisseparated i `tallization temperature of the nextsaturated componenthas been removed and we are able to obtain yields of the individualxylene isomer crystals in excess of that heretofore recoverable in knowncrystallization systems.

The mother liquor from the iirst cycle and especially from the secondcycle of para-ortho `crystallization, if the cycle is repeated, containsmeta-xylene in considerable excess of its eutectic proportion withortho-xylene, para-xylene, and/or ethylbenzene. Meta-xylene in verysubstantial yield can be crystallized from the mother liquor from eitherone or more of para-ortho crystallizations by cooling to a temperaturebetween about 90 and 110 C., and seeding with meta-xylene. Whenmeta-xylene is crystallized and separated from mother liquor after acycle of para-ortho-xylene crystallizations, the mother liquor from theAmeta-xylene crystallization can be. employed in a second para-orthocycle.

The crystallization temperatures of individuai xylene isomers or ofeutectic mixtures 4of the isomers with one another or with ethylbenzeneor other aromatic constituents of crude xylene stocks can be defined asfollows: the normal crystallization temperature is the temperature understable equilibrium conditions at which a single isomer or eutectic willcrystallize in the absence of extraneous liquids which would serve tolower the crystallization temperature; the equilibrium temperature isthe temperature at which a single isomer or a eutectic mixture willcrystallize under stable equilibrium conditions in the presence ofethylbenzcne or paraiiins or other liquid which will lower thecrystallization temperature of the yisomer or eutectic; and thespontaneous crystallization temperature is the temperature at which anindividual isomer will crystallize regardless of its tendency tosupercool or to exist as a liquid in a meta-stable condition. Thespontaneous crystallization `temperature is considerably lower than theequilibrium temperature-for the same mixture, since spontaneouscrystallization usually does not occur until after all possibilities ofsupercooling or of maintaining a meta-stable condition have beenexhausted. Of the xylene isomers, para-xylene has the least tendency andmeta-xylene the greatest tendency to supercool (to remain in meta-stablesolution over the widest temperature range).

In addition to the above intermittent fractional crys- -tallization ofmeta-xylene from the mother liquor which can be employed after one ormore cycles of para-orthoxylene crystallizations, the meta-xylene canalso be removed from the crude xylene mixture by extraction with HF-BFscomplex at any time in the process of separation of the isomers. Forexample, the crude Ca aromatic mixture can be introduced directly into aHF-BFs extraction unit wherein the meta-xylene will be removed attemperatures below 25 C. and above --40or C. At more elevatedtemperatures, disproportionation of components of the xylenes mixtureVwill occur. The mixture from which meta-xylene has been extracted isthen passed to a para-xylene crystallizer in which the mixture is cooledto a temperature below about 68 C., the normal orthopara eutecticcrystallization temperature, but above about 76 C., the spontaneouscrystallization temperature of ortho-xylene in the mixture.l Thepara-xylene is then separated from the solution by centrifugation,filtration, settling, pressing, or the like. The mother liquor is thenpassed to a second crystallization where it is maintained at above about74 to 80 C., which is approximately the spontaneous crystallizationtemperature of the paraxylene remaining in the mixture. The mixture isseeded with ortho-xylene, and ortho-xylene is crystallized from themother liquor and is separated therefrom. Here again the cycle ofparaand ortho-xylene crystallization Vcan be repeated. Ethylbenzene maybe recovered and puried by distillation of the remaining mother liquorafter one or more cycles of para-ortho crystallization.

In operation of the xylene`isomer separation process of the presentinvention, cooling ofthe xylene feed can 4 be provided by a multiplestage system refrigerated, such, for example as that diagrammaticallyillustrated in the accompanying drawing. The feed stock to this systemgenerally will not vary greatly in relative concentration of the xyleneisomers from their thermodynamic equilibrium concentration (Bureau ofStandards; Journal of Research, 39, No. 4, pp. 303-308 (1937)) and willcontain about 10 to 20 percent of para-xylene, about 15 to 25 percent ofortho-xylene, about 30 tot 40 percent of meta-xyleue, about 20 to 30percent of ethylbenzene, and about 5 to 10 percent of paraflins and atrace of Cs aromatics.

The feed stock is flowed from a source 10 and is pumped by pump 11through line 12 into a drier 13 in which it is dried with, for example,calcium chloride. The dried feed ows from drier 13 through a line 14into a cooler 16 in which the feed stock is cooled to a temperature thatis just above the highest spontaneous crystallization temperature of theother components in the mixture, in the present case between 75 and 90C. The cooled mixture is owed from the cooler 16 through line 17 intopara-xylene crystallizer 18, which is preferably an Oslo-typecrystallizer having baffles 19 beneath which a slurry of para-xylenecrystals settle. The said slurry is withdrawn through valved line 20 andis introduced into centrifuge 22 in which the para-xylene crystals areseparated from mother liquor and are washed in a fresh feed, meltedpara-xylene or an extraneous washing liquid introduced into thecentrifuge through line 21. Para-xylene product is withdrawn throughline 23 to storage or further purification means (not shown). Motherliquor can be withdrawn from the settling zone beneath the bafes 19 andowed through line 24 to line 26. Mother liquor is also withdrawn fromthe centrifuge 22 through line 26 and is pumped by pump 27 through line28 to cooler 29 and line 30 into crystallizer 31, which is ofsubstantially the same construction as that of crystallizer 18.Crystallizer -31 may also be provided with baliies 32.

Mother liquor from the first crystallization step is cooled in cooler 29to a temperature between about and 95 C. and is seeded in thecrystallizer with ortho-xylene crystals introduced through inlet 31a. Aslurry of crystals and mother liquor is withdrawn from crystallizer 31through valved line 33 into centrifuge 34 in which the ortho-xylenecrystals are separated from mother liquor and withdrawn through line 36to further purifying means, to storage, or to synthesis processes, suchas the production of phthalic anhydride, directly employing theortho-xylene. The crystalsv can be washed in the centrifuge 34 by freshfeed, liquid ortho-xylene or some extraneous wash liquid such astoluene, introduced therein through line 35. Mother liquor is withdrawnfrom crystallizer 31 through line 37 and from centrifuge 34 through line38 and is pumped by pump 39 through valved line 40 into cooler 41. rIhemother liquor is cooled to a temperature between about- 98 and 104 C. inthe cooler 41 and is withdrawn therefrom through line 42, is introducedinto crystallizer 43 having internal baies 44. The cooled mother liquorin crystallizer 43 is seeded with para-xylene crystals introducedthrough inlet 43a and the resultant slurry of para xylene crystals andmother liquor settles in crystallizer 43 and is withdrawn therefromthrough valved line 46. The slurry is introduced from the valved line 46into centrifuge 47 wherein the para-xylene crystals are separated fromthe mother liquor and are washed in the manner hereinbefore describedfor the rstrecovered para-xylene. The para-xylene crystals are withdrawnthrough line 48 to storage means or the like.

Mother liquor from this para-xylene crystallization withdrawn frombeneath balies 44 in crystallizer 43 and from centrifuge 47 through line50, is pumped by pump 51 through .line 52 into cooler 53. The motherliquor Yfrom this second para-xylene crystallization is cooled in thecooler 53 to a temperature between about -lQS and 110 C. andis thendeliveredlthrough line V54 into crystallizer 56 having internal batlles57. The cooled mother liquor in crystallizer 56 is seeded with ortho-Xylene crystals introduced through line 56a and the lresultant slurryyof ortho-Xylene crystals and mother liquor lsettles in the crystallizerand is withdrawn therefrom through valved line 58 into centrifuge 5.9.The ortho-Xylene crystals in centrifuge 59 are separated from the motherliquor and are washed in the manner described for the first-recoveredortho-xylene and the separated and Washed ortho-Xylene is Withdrawnthrough line 6i) to storage means or the like (not shown). Mother liquorwithdrawn from crystallizer 5,6, from beneath baffles 57 through line 61and from centrifuge 59 through line 62, is pumped by pump 63 throughvalved line 64 into cooler 66. h

Mother liquor from the second ortho-xylene crystallization is cooled inthe cooler 66 to temperature between about 90 and 110V u l through line67 into crystallizer 68 having internal bales 69. The cooled motherliquor is seeded, in the crystallizer, with meta-xylene crystalsintroduced through inlet 68a. A meta-Xylene crystal slurry settles inthe crystallizer 68 and is withdrawn through valved line-70 tocentrifuge 71 in which the meta-Xylene crystals are separated from themother liquor, and are'washed by liquid meta-xylene oran extraneous washliquid. Meta-Xylene crystals are withdrawn through line 72 to storagemeans or the like. from crystallizer 68 and through line 74 fromcentrifuge 71 is pumped by pump 76 through valved line 77 either toother crystallization stages, crystallization means for the recovery o fethylbenzene, or to storage.

In an alternative system, meta-xylene can be crystallized from themother liquor after the rst cycle of paraand ortho-xylenecrystallization. In this case, the valve 40a in valved line 40 is closedand mother liquor from the rst ortho-xylene crystallization is flowedthrough valved line 78 into cooler 66 and thence through line 67 intothe crystallizer 68; The cooled liquor in crystallizer 63 is seeded withmeta-xylene and resultant meta-xylene crystals are separated in theusual manner. Mother liquor that is separated in the crystallizer 68 andcentrifuge 71 is pumped by the pump 76 through valved line 79 to cooler41. Para-xylene and ortho-xylene are then each crystallized from themother liquor in an additional cycle of paraand ortho-Xylenecrystallizations.

The mother liquor remaining after crystallization of para, ortho, andmeta-Xylene according to our described method will contain at most about3 to 5 percent paraxylene, 8 to 10 percent ortho-xylene, and less than2O percent meta-xylene.

Example 1 A feed stock consisting of a distillate fraction ofr Cshydrocarbons containing 16 volume percentk ortho-Xylene, 33` volumepercent meta-xylene, 14 volume percent para- Xylene, 27l volume percentethylbenzene, and l0` volume percent parans, was introduced into acoolerwherein the refrigeration was supplied by indirect cooling withliquidair. Crystallization was performed in six stages, the first twostages consisting of a cycle of para, ortho- Xylene crystallizations inwhich the feed was first cooled to substantially the spontaneouscrystallization temperatureoof ortho-Xylene, and para-Xylene wascrystallized and separated, and in which the mother liquor from thee'rststage was cooled to 93.5 C. and was seeded with `orthoxylene.Ortho-Xylene was crystallized and separated. The mother liquor from theortho-Xylene crystallization was cooled to -899 C. and was seeded withmeta-xylene, which crystallized and was separated. The para, orthoxylenecrystallization cycle was then repeated, followed by a sixthcrystallization step in which rneta-xylenewas separated by cooling andseeding at a crystallization tem C. and is flowed therefrom Motherliquor withdrawn through line 73 rereture of -1100 Crystalli- Percent ofCumulative Stage No. Isomer CryszationTem- Isomer Percent oi Y tallizedpenrture, Recovered Isomer C. Recovered Para-xylene. -89 79 79Mota-xyleue .-189 43 43 Para-nylons. -,.98. 5 50 89 Ortho-nylene.. -10350 73 Meta-xylene 47 70 Mother liquor `from the sixth crystallizationstage contained 8 volume percent of ortho-xylene, 19 volume'pe'rcent ofmeta-Xylene, 3 volume-percent of para-Xylene, 5l volume percent ofethylbenzene, and 19 volume percent of paraflins, this representing arecovery shown in the cumulative column above 89 percent of theparaxylene, 73 percent of the ortho-Xylene and 70. percent of themeta-xylene. It can be seen from the above table that the yield ofpara-xylene was increased by our system from 79 percent (the totalamount that can be recovered by supercooling, i. e., cooling down to thespontaneous crystallization temperature of ortho-xylene) to 89 percent,an increase in recoveryA of about 10 percent.

Equipment can bo provided between the orystall-izers and after each`centrifuge 12,0. filter the mother liquor flowing therebetween sofas toinsure the elimination of erystals from the liquor, wliieh crystalsmight otherwise interfere with proper seeding and crystallization in theSubsequent Crystallization Step. However. in the usual course. of.Yoperation. the mother; liquor willi be. warmed enough by passingtlirtmghl the eentrttugefandi subsequent lines, to dissolve; anyYCryst-els which may have, passed through the` centrifugeA Alternatively;war-mine means can be supplied to ensure the solution of any such;Crystals in the. mother; liquori@ Examzzle 2 In this example a sample ofthe product of a hydroformer operation employing Cs hydrocarbonsr in thefeed was used.- This feedstock contained xylene isomers in theproportions set forth in the following table:

Three portionsof the mother liquor were cooled` to` a temperature of -91C and the separate portions were held respectively for periods of. tiveminutes.,y twohours, and threehours and thirty minutes. The crystalswerefound in each to be substantially pure, para-xylene only a trace-ofother-isomerbeingypresent.` Nomarked effect from changingthe holdingperiod before separation; of crystals from, mother liquor was observed".The mother liquor from the three` samples o f feed employed in thecrystallization of para-Xylene was. combined. Que por,- tion ofthecombined mother liquor was cooledto a. temperature of about .-89.3 C.andanother. to -949 C., each, portion was seeded by adding a. crystal ofortho- Xylene and. 26 percent and. 46 percent, respe,ctively oftheoretically availableortho-xylene was obtained. The Ofrthcfxyleue wassubstantially pure. The; ortho-Xylene was carefully filtered. from; bothportions. of the mother liquor, the` mother liquor wasv combined;cooledstill; further; to -l(l0 C.,wseeded` withpara-xylene and heldtfora period of 20. minutes. whereupon a. 23. percenty additional yield.- ofsubstantially pure parafxylene, crystals lwas recovered, a` cumulative.yieldi of 87. percent. of. the. total theoretically availablepara-xylene. The mother liquor from the para-xylenev crystallizationcontained metaxylene in such concentration as to have an equilibriummelting temperature of 81 C. By seeding the separated mother liquor withmeta-xylene and cooling to about 100 C. to 110 C. a substantial portion,about 75 percent, of the theoretically available metaxylene wasrecovered.

Example 3 A crude mixture containing 16 volume percent orthoxylene, 23volume percent meta-xylene, l-'tvolume perv cent para-xylene, 27 volumepercent ethylbenzene and 10 volume percent parains was extracted withHF-BFa and 95 percent of meta-xylene was removed in substantially pureform. Para-xylene and ortho-xylene were then crystallized from theremaining solution in 4 stages, or 2 cycles, according to the tablegiven below:

The mother liquor after the second cycle of crystallization contained11.8 volume percent ortho-xylene, 4.3 volume percent meta-xylene, 3.9volume percent paraxylene, 58.4 volume percent ethylbenzene, and 28.6percent parafns. Such mixture can be distilled to provide a highlyconcentrated ethylbenzene and a xylene-rich fraction. This xylene-richfraction may be returned to the HF-BF3 extraction unit and then berecycled through the fractional crystallization stages in order toobtain the final separation of substantially all of the xylene isomers.

The above alternative may be advantageous where refrigeration costs aresubstantial as it may be observed in the above table the extraction ofmeta-xylene prior to fractional crystallization permits a cumulativeseparation of individual paraand orthoisomers equal to the separationthat can be obtained with meta present and at 12 to 17centigrade-degrees higher temperature.

Our process may be carried out in a succession of batch operations in asingle crystallizer with recycle of mother liquor to said crystallizer.However, the present system of crystallization is especially welladapted to continuous operation, as described in the drawing, whereincooled xylene is passed from one vessel to -another as it is separatedfrom the crystals formed during a single stage and in whichcentrifugation or filtration eiectively completes the separation ofcrystals from the mother liquor. The presence of crystals of one isomerduring the crystallization of another isomer will tend to seed the iirstisomer which would otherwise stay in the supersaturated condition in themother liquor and will contaminate the crystals of the other isomer. Forexample, when para-xylene is crystallized from a feed stock identical tothat employed in Example 2 in which ortho-xylene y crystals are at thesame time permitted to seed the mother liquor, the para-xylene purity issubstantially reduced to, for example, a purity of about 50 to 60percent.

Other methods for effecting the crystallization of supersaturatedcomponentsof a liquid than the known expedient of seeding with a crystalof the supersaturated component can be employed; the other methods caninclude cooling, and preferably shock cooling, the solution anadditional amount or subjecting the solution to violent agitations orsharp vibrations.

Having described our invention, we claim:

1. A process of recovering and separating the individual xylene isomersfrom a mixture of the isomers and other miscible hydrocarbons boilingwithin substantially the same temperature range, which process comprisesrecovering,prior to crystallization of metaxylene, paraand ortho-xylenein at least one sequence of crystallizations by crystallizingpara-xylene from the mixture by cooling it to a temperature just abovethe highest `spontaneous crystallization temperature of the othercomponents of the mixture and below the equilibrium crystallizationtemperature of para-xylene, separating paraxylene crystals from themother liquor, cooling the mother liquor to a temperature below theequilibrium crystallization temperature of ortho-xylene and just abovethe highest spontaneous crystallization temperature of the othercomponents of the liquor and seeding the mother liquor withortho-xylene, crystallizing and thereafter separating crystals ofortho-xylene therefrom, and cooling mother liquor from which para-xyleneand orthoxylene have been separated to below the equilibriumcrystallization temperature of meta-xylene and to just above the highestspontaneous crystallization temperature of remaining other components ofthe second mother liquor, seeding the second liquor with a crystal ofmetaxylene and thereby crystallizing and recovering metaxylene from themixture.

2. A process of separating and recovering individual xylene isomers insubstantially pure form from a crudeV mixture of the isomers containingfrom about 10 to 20 percent para-xylene, about 15 to 25 percentortho-xylene, about 30 to 40 percent meta-xylene, about 20 to 30 percentethylbenzene, and about 5 to l0 percent parafns boiling in substantiallythe same range as the xylenes, which process comprises cooling the saidcrude mixture to a temperature between about and 90 C. to eiectcrystallization of para-xylene, separating paraxylene from the motherliquor, cooling the mother liquor to a temperature between about and 95C., seeding the cooled mother liquor with ortho-xylene, crystallizingand separating ortho-xylene from the mother liquor,A cooling the motherliquor remaining after at least one cycle of the para-ortho xylenecrystallization to ajtemperature between about and 110 C. and seedingthe cooled mother liquor with meta-xylene to eiect crystallization ofmeta-xylene fromthe mother liquor.

3. A process of recovering and separating the individual xylene isomersfrom a mixture of the isomers and other miscible hydrocarbons boilingwith substantially the same temperature range, which process comprises-crystallizing para-xylene from the mixture by cooling it to atemperature just above the highest spontaneous crystallizationtemperature of the other components of the mixture and below theequilibrium crystallization temperature of para-xylene, separatingpara-xylene crystals from the mother liquor, cooling the mother liquorto a temperature below the equilibrium crystallization temperature ofortho-xylene and just above the highest spontanecus crystallizationtemperature of the other components of the liquor and seeding the motherliquor with orthoxylene, crystallizing Vand separating crystals oforthoxylene therefrom, and cooling resulting second mother liquor fromwhich para-xylene and ortho-xylene have been separated to below theequilibrium crystallization temperature of para-xylene and to just abovethe highest spontaneous crystallization temperature of remaining othercomponents of the second mother liquor, seeding the second liquor with acrystal of para-xylene, crystallizing and recovering para-xylenetherefrom, cooling the resulting mother liquor to below the equilibriumcrystallization temperature of ortho-xylene and to just above the1highest spontaneous crystallization temperature of remaining othercomponents of the mother liquor, seedingthe cooled mother liquor with acrystal of ortho-xylene, crystallizing and recovering ortho-xylenetherefrom, cooling the resulting mother liquor to below the equilibriumcrystallization temperature of meta-xylene and to just above the highestspontaneous crystallization temperature yof remaining other componentsof the mother liquor,

10 seeding the cooled mother liquor with a crystalof meta- 2,530,978Mason Nov. 21, 1950 xylene and thereby crystallizing and recoveringmeta- 2,622,115 Carney Dec. 16, 1952 xylene from the mixture.

OTHER REFERENCES References Cited in the le of this patent 5 McCaulay etal.: Ind. Eng. Chem, V01- 42, PP- 2103-7 UNITED STATES PATENTS (19502,398,526 Greenburg Apr. 16, 1946

1. A PROCESS OF RECOVERING AND SEPARATING THE INDIVIDUAL XYLENE ISOMERSFROM A MIXTURE OF THE ISOMERS AND OTHER MISCIBLE HYDROCARBONS BOILINGWITHIN SUBSTANTIALLY THE SAME TEMPERATURE RANGE, WHICH PROCESS COMPRISESRECOVERING, PRIOR TO CRYSTALLIZATION OF METHZYLENE, PARAAND ORTHO-XYLENEIN AT LEAST ONE SEQUENCE OF CRYSTALLIZATIONS BY CRYSTALLIZINGPARA-XYLENE FROM THE MIXTURE BY COOLING IT TO A TEMPERATURE JUST ABOVETHE HIGHEST SPONTANEOUS CRYSTALLIZTION TEMPERATURE OF THE OTHERCOMPONENTS OF THE MIXTURE AND BELOW THE EQUILIBRIUM CRYSTALLIZATIONTEMPERATURE OF PARA-XYLENE, SEPARATING PARAXYLENE CRYSTALS FROM THEMOTHER LIQUOR, COOLING THE MOTHER LIQUOR TO A TEMPERATURE BELOW THEEQUILIBRIUM CRYSTALLIZATION TEMPERATURE OF ORTHO-XYLENE AND JUST ABOVETHE HIGHEST SPONTANEOUS CRYSTALLIZATION TEMPERATURE OF THE OTHERCOMPONENTS OF THE LIQUOR AND SEEDING THE MOTHER LIQUOR WITHORTHO-XYLENE, CRYSTALLIZING AND THEREAFTER SEPARATING CRYSTALS OFORTHO-XYLENE THEREFROM, AND COOLING MOTHER LIQUOR FROM WHICH PARA-XYLENEAND ORTHOXYLENE HAVE BEEN SEPARATED TO BELOW THE EQUILIBRIUMCRYSTALLIZATION TEMPERATURE OF META-XYLENE AND TO JUST ABOVE THE HIGHESTSPONTANEOUS CRYSTALLIZATION TEMPERATURE OF REMAINING OTHER COMPONENTS OFTHE SECOND MOTHER LIQUOR, SEEDING THE SECOND LIQUOR WITH A CRYSTAL OFMETAXYLENE AND THEREBY CRYSTALLIZING AND RECOVERING METAXYLENE FROM THEMIXTURE.